Avalanche photodetectors (APDs) are used in a wide variety of applications in today's advancing optoelectronics industry. For example, avalanche photodetectors are commonly used as optical monitors and optical receivers.
When light, i.e., an optical signal, is directed to and absorbed by an absorption layer of an APD, electron-hole pairs are created and an electrical current is generated thereby converting the optical signal to an electrical signal. It is desirable to produce APDs with high quantum efficiency, high gain and high bandwidth over a wide range of bias voltages.
An avalanche photodetector includes at least an absorption layer in which light is absorbed and electron-hole pairs are produced and a multiplication region in which avalanche multiplication takes place. In the multiplication region, electrons are accelerated through electric fields, gain energy and form additional electron-hole pairs, liberating additional electrons. The multiplication region is therefore seen to be the region in which gain occurs.
Previous attempts to produce high gain, high bandwidth APDs over a wide range of bias voltages include the reduction of absorption and/or multiplication layer thicknesses to improve bandwidth and the use of a superlattice rather than a homogeneous material as the multiplication region, to enhance gain. An undesired consequence of the reduction of absorption layer thickness is lower quantum efficiency. Gain, achieved in a multiplication region, must compensate for this loss in quantum efficiency. Decreasing the width of the multiplication region to improve bandwidth performance, however, also lowers the maximum achievable gain. Neither of the two approaches of reducing absorption and/or multiplication layer thicknesses, however, address the bandwidth-limiting problems and produce an APD with a suitably high bandwidth. Although a superlattice structure achieves more avalanche gain per unit length than a homogeneous material, it is unlikely that such superlattice structures are optimal in this regard, especially in relatively short multiplication regions. This is due, in part, to the fact that electron-hole pairs do not remain in the relatively low bandgap layers of the multiplication region in a superlattice structure, long enough to exploit the increase in input ionization probability attained when an electron accelerates through the multiplication region. Additionally, the use of a superlattice structure may produce dark current effects that can adversely affect the signal.
InGaAs/InAlAs-based avalanche photodetectors are favored in today's optoelectronics communication industry because of the familiarity and availability of the materials and methods used to form such photodetectors.
It can be therefore understood that it would be desirable to produce an InGaAs/InAlAs-based avalanche photodetector with both high gain and high bandwidth over a wide range of bias voltages without the aforementioned trade-offs and shortcomings that can limit gain and/or quantum efficiency.